ACARS - un'analisi di Dennis Cimino sulla propagazione delle trasmissioni VHF a 130 MHz dagli aeroplani in volo

Ripropongo qui un fondamentale articolo di Dennis Cimino pubblicato su 911ACARS, sulla propagazione delle onde VHF da aereo a terra. Per miglior precisione dei termini tecnici e delle relative spiegazioni, mi permetto di lasciare l'articolo in lingua originale...

First, I want to state for the record that I have been working with high power R.F. transmitters and countermeasures sets, radar, communications, deception repeaters and jammers since 1973, and am quite experienced with in-flight dynamics of VHF signal propagation from both low altitude and high altitude aircraft, both civilian and military. Now, the average person is not too familiar with voltage standing wave ratios, or effective radiated power, insertion line losses, reflected power, antenna gain factors or for that matter antenna radiation patterns on aircraft, as I am. I have extensive experience in these matters and have the actual aviation experience in a wide variety of aircraft, with regard to everything from H.F. long haul communications to millimeter wave jamming or ECM systems, radar, and navigation equipment. For this discussion we are going to talk strictly about 130 MHz, not 2 MHz, not 17 GHz, and the beam pattern and directionality and effective ranges of these systems on both commercial, military and commercial aircraft both below and above the 12,500 pound weight category.

For those of you who are unfamiliar with path loss nomographs for 130 MHz, I would like you to make it a point to study the diagram below. Notice that no assumption is made for effective radiated power, or system gain factors, and there is a reason for this.

This diagram is shown to illustrate the fact that, contrary to assertions by JREF and Unexplained Mysteries bloggers, radio waves do not just "keep on propagating" outwards at their original radiated power levels, without being attenuated over distance. When the path losses and other factors exceed the transmitter power and effective radiated power, as well as minimum discernible signal sensitivity of the receiver system, on any radial from the aircraft, and any angle of incidence, there is no possible communication then. Radio signals do not keep on radiating outwards without attenuation occurring. Due to the totally unpredictable nature of the antenna radiation patterns of VHF blade antennae used by ACARS, it is a false assertion to state that there are no limitations between the RGS stations and the aircraft transmitter antenna for COM3, usually used on most Boeing airplanes, circa 1980’s and onwards, for ACARS.
In any case, what I would like to start with is that the aircraft and the antenna jointly form the transmit and receive radiation pattern in ways that are radically different from these same antenna systems being mounted on flat ground planes that offer at the very least a 90 degree counterpoise to the plane wave or electrical field leaving the antenna. The reason I cite this is that for any isotropic or non-directional antenna system, which these are not, due to their shapes being swept to minimize aerodynamic drag factors and prevent ice pickup or vibration resonances, these antennae used on commercial and military aircraft for VHF COMMUNICATIONS are what are referred to as ‘blade’ antennae of the type used on most commercial aircraft, including B-757/B-767’s.

For such antennae, there are some considerations that apply to them due to their shorter than quarter wave physical radiating length offset from the aircraft’s very curved fuselage or empennage. This results in significantly higher V.S.W.R. or ‘reflected’ energy back to the transmitter and not into free space. In addition to this, the shorter physical radiating element length versus the curved fuselage will radically change the radiation pattern and add directivity to these antennae, typically along the fuselage plane, fore and aft, as well as pushing the radiation pattern upwards or away (assuming the antenna is top mounted) so that the beam shape is not cardioid in nature but scalloped and with significant major pattern lobes that tend to create significant null zones that effectively reduce radiated power in those areas to the sides or in the ‘X’ plane (horizontal) by several ‘dB’ of gain, while significantly favoring the fore and aft orientation along the fuselage. In the ‘Y’ axis (Vertical), looking at the beam pattern, the field lobes from the antenna depart the skin of the aircraft and contour upwards, once again, assuming the antenna is top mounted, and conversely, ‘downwards’ if the antenna is belly mounted. To characterize the full radiation or beam pattern, it would appear heavily scalloped in the ‘X’ axis, and quite finger or balloon lobed in the ‘Y’ axis, (vertical) plane. In effect, the antenna that was supposed to be fairly omnidirectional now is quite directional both fore and aft and has markedly higher gain factors in those directions, as well as pushing the pattern upwards into free space at any number of angles less than 90 degrees off the horizontal. The resultant 3-dimensional plot begins to look like a twisted doggie balloon shape and no longer resembles a spherical omnidirectional beam plot.

The reason I mention this is that this directivity is nothing that can be readily compensated for without exotic counterpoise shapes around the base of the antenna, or multiple ‘Yagi’ type of elements in other directions to flatten or change the beam pattern. To compensate for these exotic and quite scalloped and lobed patterns that become reality when these relatively inefficient due to shorter radiating element length antennae are joined to a curving cylindrical tube which airplane fuselages tend to be, relative to the radiated field. And I would like to state that though there is a great deal of reciprocity between TRANSMIT and RECEIVE patterns, they do not necessarily fully reciprocate or remain the same for both functions due to any number of factors we will not discuss herein.

In any case, what I have just described is the highly irregular and moderately unpredictable and undesirable directivity of an otherwise fairly omnidirectional antenna system. It is important to note this because a 3 dBm power differential in any of these planes, radiation pattern wise, effectively is a half power addition or reduction, with significant null points deeper than this that occur quite naturally, rendering the pattern even more complex and difficult to factor using system loss calculations, without actually making measurements in every axis with the aircraft either raised up and outside of what is called ‘near field’ distances of the ground below the aircraft. Simply put, real world antenna radiation pattern measurements are possible and actually done, but in most cases scaled models with scaled frequencies, are typically used on non full-sized models inside of anechoic chambers or OTS (outdoor test) areas. By large, most aircraft manufacturers do not attempt to make these measurements but make assumptions about radiation patterns that are not necessarily founded in known factual testing data, as usually is derived by MILITARY aircraft certifications laboratories who have a much more vested interest in characterizing these radiation patterns to maximize system effectiveness and head off or prevent unwanted ‘dead zones’ around the fighter or bomber or jamming platform these antennae are mounted on. On many tactical jamming aircraft, the blade antenna arrays are more or less in ‘Yagi’ configurations to give directional gain that the mission capability of the aircraft needs to make maximum effective use of a communications jammer mounted on the aircraft. This is, however, never done on commercial airliner or Part 23 aircraft, to minimize cost and reduce weight.

As we have now more or less just barely touched on the radiation pattern, we have not yet begun to talk about signal polarization issues, and why they go hand in hand with more fully understanding how a theoretical two-way path loss calculation and predicted range of a system can be quite off in comparison to real world or ‘true’ and valid path losses and system effective radiated power determination, or receive sensitivity calculations.

What I am getting here is that a perfect isotropic vertical radiator element that rises 90 degrees off of a ground plane, or is oriented in the pure ‘Y’ axis pointed straight up, effectively has a VERTICAL ELECTRICAL FIELD POLARIZATION OF THE SIGNAL. In lay-person’s terms, ‘polarity’ of the electrical field, ideally, should be the same on both the transmit and receive end, to maximize signal transfer. In real life, this is difficult to achieve with swept blade antennae and curving aircraft fuselages which are the ground plane or ‘counterpoise’ for the antenna itself. The resultant polarization skewing hence causes a coupling factor inefficiency to occur, relative to the very polarization fixed antenna on the ground, intended to receive these signals.
IF the electrical field polarization shifts fully 90 degrees so it is not vertical at all but more ‘horizontal’ in relation to the very VERTICAL ground antennae respective polarity, the coupling mismatch is now at the very least ‘6’ dB less than optimal, of not more due to the curving fuselage, which is not a flat ground plane at all. Every ‘3’ dB is a half power loss when that number is a negative change. Why is this important?

Let’s theorize that from the transmitter rack, of the 50 watts transmitted off the jack in the rack itself, after ‘xx’ dB of loss in the transmission line, which in some cases is several meters in length, we are not going to see 50 watts of R.F. energy radiated into free space off of this antenna. Instead, if we are lucky, we’re more likely to see far less than 20 watts or thereabouts, leaving in all directions, not just going out in a single plane wave directly at the receive system on the ground, miles below. To better visualize this, imagine a light bulb on a lamp without a shade radiating light energy in all directions more or less evenly. Though light energy is significantly higher in frequency than is 130 Mhz. ACARS data link signal data, imagine the light bulb now has a series of fiber optics tubes radiating in all directions, affixed to the light bulb. In effect, the result is not anywhere near an omni-directional pattern at all, but a highly irregular, porcupine spoked shaped pattern of peaks and nulls radiating outwards, resulting in shadows in all directions on the walls of the room, and bright spots. This is more or less what the irregular radiation pattern of the ACARS blade antenna would look like if this were optical energy at a higher frequency. This analogy, as coarse as it may seem, is more or less descriptive of R.F. power density variations from a very spiked and valley ridden pattern radiating off the fuselage from the ACARS blade antenna, typically mounted on the aft belly of the fuselage on some Boeing aircraft, such as is the case with the B-737NG.

Why is this important to understand, relative to the ACARS two-way range issue? Well, simply put, the moderately less than optimal V.S.W.R. of the antenna and transmission line and airplane fuselage now has reflected or ‘lost’ significant amounts of outbound R.F. energy that would have gone into free space in any number of ‘X’ and ‘Y’ and in fact, ‘Z’ plane directions from the radiating antenna or ‘blade’ on the fuselage. So we’re not dealing with our original 50 watts any longer, we are now pushing less than half of that into the free space around the airplane, in virtually all directions, in a very far from omnidirectional radiation pattern, with deep peaks and nulls radiating outwards in all directions but overall favoring the fore and aft longitudinal axis of the aircraft. Translated, it means the myriad of ground stations around the aircraft will not be seeing anything close to the optimal range nomograph numbers at all, based on free space losses, as well as the very irregular antenna radiation pattern of the aircraft’s VHF blade antennas that are used by ACARS.

So now we are going to bring up the 200 mile claimed range number here. And the reason why, is that yes, in theory, due to the minimum discernible signal or sensitivity level of the ACARS receiver system of both the plane and the ground station is more or less known or can be approximated, what cannot be known is the coupling factor perturbations due to beam shaping and irregular radiation patterns that occur when blade antennas used in VHF communications are mounted on tubular aircraft fuselages. This results in significantly ‘lower’ than the expected or theoretical best 200 mile range supposition, by as much as 80 percent in real life.

Any experienced aviator who has used VOR navigation and who understands how the F.A.A. determined SERVICE VOLUME models for the each class of VOR facility, knows that even at FL-450, and above, a realm that few airliners routinely will fly at, by the way, due to cabin pressurization issues and aerodynamics, the effective range of the slightly lower in frequency of 108 to 118 Mhz., is reduced from that altitude to FL-600 or sixty thousand feet. Why is this? Well, ground obstructions, and earth curvature now come into play here, as well as other similar signal degrading changes, and therefore the F.A.A.’s own data available to look at for these VOR facilities is less above FL-450. The F.A.A. didn’t arbitrarily pick these numbers out of some hat somewhere. They were derived by in-flight testing over literally all terrain types and at all altitudes, inclusive of FL-600 in some cases. The service volume diagrams from the F.A.A. website are relatively transferable to ACARS due to similarity of known and demonstrated effective ranges derived thru years of collective experience testing VOR reception service volumes, as well as R.C.O (remote communications outlet) communications ranges.

So we have more or less touched on irregular and highly unpredictable antenna radiation patterns in both the transmit and receive capability of 130 Mhz. communications and navigation systems in use today, and for all practical purposes really destroyed the rather inconsistent with real world performance, claims of ACARS working out to 200 nautical miles from any RGS station, due to these factors. In real life, no experienced line pilot could every with a straight face claim that he’s tracked VOR’s at 200 nautical miles without flag tripping and sporadic reception, but also, it would be quite a stretch of the imagination to expect the performance for ACARS to routinely, every time, ever truly meet the outrageous claims of functionality at 200 miles from any aircraft, due to these factors cited here. Power output is not the determining factor here. Earth curvature is the predominant range limiter, as is terrain irregularity and obstructions around the ground station inside both ‘near field’ and ‘far field’ distances from the RGS stations.

As a commercially rated pilot with experience flying at virtually all of these altitudes up thru at least FL-350 in civil and military aircraft both as pilot and test engineer, I have to stipulate here that it is both ‘laughable’ and quite ‘uncommon’ to expect VOR reception much beyond 135 nautical miles even at the higher altitude regimes, and for that matter, to expect ACARS to work much further than these ranges over hilly and irregular terrain with mountains and ridges everywhere to throw a major ‘curve’ into any range calculation for these ACARS path losses.

Unfortunately many people who are claiming expertise in these areas are neither qualified to make these judgments due to a lack of R.F. engineering experience or real world testing experience and backgrounds, nor do they have any real world aircraft flight testing experience or flight experience using these systems in the real world. To assert that ACARS can communicate at 200 miles is not factual nor is it realistic. And it is wholly unfounded by any known factual testing data or practical experience of these outrageously optimistic ranges. Furthermore, I’ll even go one step further here. If ANY experienced and non-anonymous airman with a valid pilot certificate at COMMERCIAL level and decades and thousands of hours of flight experience can validate VOR receptions beyond 150 miles under any circumstances, I would like to see their data and where this took place, in which aircraft, at what altitude. I do not expect many challenges to this more or less hard and fast rule here for VHF being pretty line-of-sight limited with very little effective terrain contouring or tropospheric ducting taking place. Any reasonably experienced line captain who has extensive ACARS experience in the Positive Controlled Airspace is welcome to controvert these claims with hard factual data.

In summary, the radiation patterns of real VHF communications blade antennas on heavy transport category aircraft, are both complex and highly unpredictable except to state that nominally there is some beam pattern lobing that prefers the longitudinal axis of the aircraft, to the fore and aft directions, while also shifting the electrical field far from optimal vertical polarization into any number of less optimal, and very inefficient signal polarizations that result in reduced ranges in most directions, and predictable and even more complex changes when these aircraft roll and bank in turns, further altering their radiation and reception patterns significantly from straight and level flight. Furthermore, as the aircraft’s crew is quite unaware this is in fact occurring, no action can be taken by either the crew of the aircraft’s system that can compensate for this. It is out of their hands entirely.

For the most part, it is unclear why some would surmise that ACARS should always be able to realistically and effectively work out to 200 nautical miles, and furthermore, that stations significantly closer to the aircraft would be totally unseen and not communicating under any circumstances, given what has been discussed here. Yes, the radiation pattern is lobed in all ‘3’ axis off of the aircraft, and yes this means the signal will follow the ground plane of the fuselage and wing structures somewhat, but to make the assertion that a station more than 100 nautical miles away would be favored over one that is less than 20 miles away, is not genuine nor credible.

We did not discuss a whole slew of other factors that adversely impact the effective ranges, such are atmospheric noise sources, co-channel interference, and things such as multipath and reflections from both near the RGS station as well as the aircraft’s structures itself. Even something such as flap extension or landing gear deployment will significantly change the radiation patterns of most aircraft, whether the aircraft design or ARINC radio engineers like that or not.

ACARS is not likely to work much further than 150 miles in most cases, under any and all circumstances, due to very extensive and recognized use of VOR changeover points on LOW ALTITUDE enroute IFR charts, showing that even at low altitudes up to and inclusive of FL-180, various enroute or Victor Airway reception ranges have been know for years and are published in both U.S. Government and Jeppesen/Sanderson IFR enroute charts and terminal charts for very very good reasons. These changeover points are the very best illustration of actual line-of-sight VHF communications ranges being adversely impacted by hilly terrain and other issues much more so than altitude, because if altitude above the ground were the sole arbiter of this, all changeover points for these low altitude airways or Victor Airways would be the same every single time, but they are not.

They are individual as hairs on your head. And so are the radial distances and ranges from any ACARS ground station using VHF line-of-sight frequencies. By necessity the technical assessment offered here is significantly less technical for a reason. You don’t need to be a pilot or a radio engineer to grasp most of these concepts explained within this moderately brief attempt to illustrate the point that several things are always going to be in play with radio ranges. Those are, to sum them up:

Radio signal strength decay over distance due to atmospheric attenuation and power density diminishment due to radial distances that dictate that with ‘X’ watts of energy radiated outwards from any radio source, that the power density window decreases on any given point far from the origin, even with highly directional and focused beams of radio energy.

Aircraft radiation patterns are not by their nature remotely close to omnidirectional at all, even in the best cases such are transponder antennae at 1030 and 1090 Mhz, perhaps one of the shorter stub blades in use on any airplane, requiring far less ground plane uniformity to properly form an omnidirectional radiation pattern.

Curvature of the earth and ground obstructions and line-of-sight VHF propagation characteristics are also significant range limiters at 130 MHz.

RGS stations an order of magnitude CLOSER to distant ones are far more likely to be the best LINE OF SIGHT paths under any and all but the most unusual cases where a mountain or a building between the RGS and the plane may in fact really make a much bigger attenuation factor between a closer station perhaps and a more distant one with a very clean, very direct, non-obstructed path, e.g.: if the more distant station is 100 or more miles away, it’s a real fantasy to state that one a mere 7 to 10 miles away, almost directly under the plane itself, would not be the preferred station, based on path losses alone, and virtually zero earth curvature to deal with there.

****
And added here as a side note.

The one thing that most lay-persons do not experience in real flight regimes is loss of both communications signals and navigation signals, with something as simple as a small heading change. Something that typically is not a factor on most civilian air carrier aircraft but has been a major problem with general aviation airplanes, is antenna position due to other structures like landing gear, exhaust systems, and other small protuberances on the airplane that really skew the signal so much that it necessitates the pilot change course or heading to blindly fish around and find a better angle offset to allow both ATC to hear you, or for your transponder's signal to be seen by a distant radar station, because your exhaust system or landing gear (in some cases always down because it is not retractable) gets in the way and changes things.

High altitude planes don't typically drop gear and flaps, but they do have many other shape perturbations in both the near field and the far field regions of these antennae that make it unlikely that the aircrew is going to alter course or turn to better line up an antenna pattern 'lobe' so the ground can now see their signal and communicate.

After years of flying both fixed gear and retractable gear planes with all sorts of shapes, I have found that antenna radiation patterns are very very spooky and variable and are not published in any flight manual. But any pilot who knows about near field and far field interference and blocking of his signal would, like me, change course to try to get the signal reception needed for a little bit longer at the extreme outer effective ranges of radar, transponder, and communications / navigation equipment. Airline crews don't do this. But over three decades of flying, I have done it routinely to regain IFF to RADAR functionality, or DME functionality, and for that matter, even COMM/NAV functionality. In one case due to the location of a pair of GPS antennae on the skin of a plane, even GPS was affected somewhat, and it changed RAIM factor on approaches...due to the ground based WAAS augmentation being less optimal on some IFR approaches.

In any case if I find a good VHF radiation pattern plot to ship, I'll send it. But these patterns are very complex. Very very complex, and in real life, as airplane makers don't get real data about this, very highly unpredictable and way less than optimal per engineering calculations for them.

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Dennis Cimino
- Electrical Engineer
- Commercial Pilot Rating, since 1981 , IFR, MEL ratings since 1980.
-Navy Combat Systems Specialist: RADAR, ECM, cryptographic communications Navy EMI troubleshooter for COMNAVSURFPAC via MOTU-5, San Diego, CA., and under NAVELEX contract. AN/SLQ-32(V)3 Countermeasures System support specialist.
-Flight Data Recorder Engineer Smiths Aerospace (Now G.E. Aero)
- BA-609, IDARS, Military and Commercial
- Millimeter wave RADAR and countermeasures expert since 1973
- Two patents held for high powered modulator Triton HP, C-Band, 250kw LONG RANGE Doppler RADAR ( Kavouras ):
- long pulsewidth RADAR droop compensation network that improved radar output power thru long pulse transmissions, effectively imrproving weather phenomena detection ranges.
- and wave guide arc detection for high powered RADAR system

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